Hybrid electric vehicles (HEV's) utilize a combination of an internal combustion engine with an electric motor to provide the power needed to propel a vehicle. This arrangement provides improved fuel economy over a vehicle that has only an internal combustion engine. One method of improving the fuel economy in an HEV is to shutdown the engine during times that the engine operates inefficiently, and is not otherwise needed to propel the vehicle. In these situations, the electric motor is used to provide all of the power needed to propel the vehicle. When the driver power demand increases such that the electric motor can no longer provide enough power to meet the demand, or if the battery state of charge (SOC) drops below a certain level, the engine starts quickly and smoothly in a manner that is nearly transparent to the driver. Another method of improving the fuel economy in an HEV is to only operate the engine in a high efficiency range, and control the motor to add power, or subtract power from the overall power needed to meet the power demand.
A modular hybrid powertrain (MHT) is a powertrain that includes components from a conventional vehicle (e.g., the engine, transmission gear box and differential) and integrates hybrid components (e.g., motor, high voltage battery, clutches) to provide a HEV.